1. Field of the Invention
The present invention relates in general to a drive circuit for an integrated circuit tester for providing a differential test signal and in particular to an inhibitable, continuously-terminated, differential drive circuit.
2. Description of Related Art
An integrated circuit (IC) tester tests switching speed and propagation delay times of an IC by transmitting a sequence of test signals to input/output (I/O) pins or terminals of the IC device under test (DUT) and measuring the timing and logic levels of output signals the DUT produces in response to the test signals. A typical integrated circuit tester includes a set of "pin electronics" circuits, one connected to each pin or terminal of the DUT. Each pin electronics circuit is capable of either sending a single-ended test signal to the DUT pin or of sampling a single-ended DUT output signal produced at the pin to determine the output signal's logic state. In order to measure response times of a DUT with a high degree of accuracy a pin electronics circuit should be able to produce a test signal with accurately timed edges. When the pin electronics circuit is sampling a DUT output signal, the drive circuit within the pin electronics that generates the test signal is turned off and put into a high-impedance state (tristated).
Each pin electronics circuit is connected to a DUT pin through a transmission line which should convey the test and DUT response signals between the pin electronics circuit and the DUT pin with as little distortion as possible. To limit transmission line distortion when the pin electronics circuit is sampling the DUT output signal, the transmission line is terminated at its pin electronics circuit end by the transmission line's characteristic impedance.
Since most integrated circuits use single-ended input and output signals, IC testers typically employ pin electronics circuits producing single-ended test signals. When a DUT requires a differential signal input, an IC tester typically employs two pin electronics circuits operating 180 degrees out of phase with one another to provide a "pseudo-differential" test signal simulating a true differential test signal. However it is difficult to match the phase of the two pin electronics circuits so that edges in the signals produced by the two pin electronics circuits precisely coincide. It is also difficult to precisely match the slew rates of the two signals to ensure that crossing occurs at the correct voltage level. As signal frequencies increase, timing errors resulting from small mismatches in behavior of the two signals forming a pseudo-differential test signal become increasingly significant.
To avoid problems associated with pseudo-differential test signals, it would be desirable to employ pin electronics circuits providing true differential test signals when testing ICs having differential signal inputs. Although differential drivers are well-known and commonly employed in the electronics industry, they have not typically employed in IC tester pin electronics circuits because it is difficult to quickly switch a differential driver to a high impedance state when the pin electronics circuit is sampling a DUT output signal instead of producing a test signal. It is possible to use a transistor or relay switch to isolate a driver from the transmission lines, but transistor switches can distort the test signal and relay switches are too slow to be used in high speed testing.
When the pin electronics circuit is receiving and sampling a DUT output signal, a transmission line should deliver the DUT output signal to the pin electronics circuit with as little distortion as possible. To help minimize signal distortion, the transmission line should be terminated at the pin electronics circuit with the transmission line's characteristic impedance. What is needed is a drive circuit that can produce a true differential test signal when needed but which provides proper transmission line terminating impedance and when the transmission line is conveying a DUT output signal.